Paleontologists have created detailed three-dimensional images of evolution's first multicellular creatures in their embryonic stages, some so detailed that they reveal more about the development of long-extinct creatures than scientists know about their modern counterparts

Team of Chinese, Swedish, Swiss and British scientists repeatedly scanned tiny balls of fossilized cells with powerful X-rays and then used a computer to assemble the views into microscopic CT scans.

Some of the embryos exhibit hitherto unknown mechanisms of embryonic development that have since gone extinct. Others have combinations of traits that put them near the lowest branches of the animal kingdom's evolutionary tree.

"The results are truly orgasmic," said Philip C. Donoghue, a paleontologist at Bristol University in England who led the team that created the images.

The pictures appear in Thursday's issue of the British journal Nature.

Paleontologists have been using acid to dissolve the embryo fossils out of rocks for about a decade, focusing on specimens from the time about 500 million years ago when multicellular animals began to proliferate. Tons of rock must be dissolved to retrieve a few hundred embryos, none of which are more than a fraction of a millimeter across.

Until now, Donoghue said, the specimens have mostly been treated as "curios of fossilization."

Using the new technique, he and his colleagues have been able to create cutaways, cross-sections and, by stringing together images of embryos at different stages of development, virtual time-lapse sequences of the animals' metamorphosis.

"It's an interesting technique," said Andrew Knoll, a professor of natural history at Harvard University. "You get an improved picture of what animal life was like in its earliest days."

The images show that one fossil embryo known by the scientific name Markuelia must be most closely related to a modern group of marine invertebrates known as the penis worms, based on the number of teeth it has and the way they are arranged.

The creature sits very near a three-way split on the evolutionary tree that separated the unsegmented nematode worms and their segmented cousins from the gigantic arthropod phylum, which includes crustaceans, insects and spiders.

Another image shows that a segmented creature known as Pseudooides had a very unusual means of assembling itself. Modern segmented animals either develop all of their segments early and then simply get bigger, or they grow by adding segments to their hind ends.

But Pseudooides added its segments in the middle, "which is really totally bizarre," Donoghue said.

Interestingly, paleontologists have yet to find any definitive examples of larval stages among their microfossils. That suggests either that larvae are especially difficult to fossilize or that development through juvenile stages developed later in evolutionary history